We have gradually developed the proposal that the airway maintains a special program for host defense and that changes in this same program may lead to airway disease. Our progress on this award extends this concept to the development of mucous cell metaplasia in chronic airway disease at two levels. First, at an upstream level, we have identified a new immune axis that features IL-13-production by lung macrophages as well as T cells. Second, we have also identified a downstream mechanism that translates the IL-13 signal into mucin gene expression. This mechanism features hCLCA1, the first member of a newly defined family of "chloride channel calcium-activated" (CLCA) proteins in humans. It is possible that additional CLCA family members may mediate airway hyperreactivity, so this is a secondary endpoint for our studies. However, the focus of our aims is to define the molecular mechanism for hCLCA1 regulation of mucous cell metaplasia. In that regard, our preliminary studies suggest that: (1) expression of hCLCA1, among the four hCLCA family members, is selectively regulated by IL-13;(2) hCLCA1 expression is sufficient for MUC5AC, MUC5B, and MUC19 mucin gene expression in human airway epithelial cells;(3) hCLCA1 appears to signal since MAPK13 becomes activated in hCLCA1-expressing cell cultures, and this activation is necessary to drive mucin gene expression. In mice, there appear to be as many as eight mClca proteins (mClca1-8) that may each be sufficient for mucous cell metaplasia, so that loss of an individual mClca can be compensated in vivo in this species. Nonetheless, in patients with chronic mucous cell metaplasia due to COPD, expression of hCLCA1 and activation of MAPK13 are found in concert with IL-13 production and mucin gene expression. Based on these findings and others, we propose a new scheme for chronic mucous cell metaplasia in which overproduction of IL-13 leads to hCLCA1 expression that drives MAPK13 activation and then mucin gene expression. We are particularly focused on defining the hypotheses that: (1) hCLCA1 is a signaling protein that translates IL-13 stimulation into mucin gene expression and a transition to mucous cells;(2) hCLCA1 function depends on the capacity of a mature hCLCA1 peptide fragment to activate MAPK13 and in turn regulate mucin gene expression;and (3) inhibition of hCLCA1 function will selectively block mucous cell metaplasia. This scheme for hCLCA1 (and orthologous mClca) control over epithelial behavior will be tested in isolated human airway epithelial cells as well as mouse models and patients with mucous cell metaplasia due to asthma or COPD. A full understanding of this pathway will provide multiple targets for therapeutic intervention, including prevention of hCLCA1 expression, processing, binding, and signaling. PUBLIC HEALTH RELEVANCE. Our research is aimed at understanding and preventing the excessive mucus production that is found in common airway diseases such as chronic obstructive pulmonary disease (COPD) and asthma. At present, no treatments are available that can selectively and effectively treat mucus overproduction. We focus on the molecular pathway leading to the development of cells that produce excessive mucus in the lung. Our work will thereby provide new therapeutic strategies to restore normal lung architecture and correct mucus production to a level that is helpful rather than harmful for lung function.